The steel sheets which are used for outdoor building materials (for example shutters, rain doors, doors, roofing materials, siding materials and the like), external fittings (for example external air conditioning motors) and the like are such that the surface of the coated steel sheet is readily stained as a result of the effects of rain (acid rain), dust storms and the like and so excellent staining resistance is required. A paint composition with which paint films which not only have staining resistance in a short period of time after coating but with which the staining resistance is maintained over a long period of time is required to provide the coated steel sheets with staining resistance.
A paint composition which includes a silicon oxide sol of which the surface has been modified with a silane coupling agent which has a specified average particle diameter has been disclosed in Patent Document 1 as a paint composition which has such staining resistance. With a paint film obtained by coating this paint composition the staining resistance of the paint film is retained for a long period of time but the period of time after coating until staining resistance appears needs to be shortened.
Furthermore, a surface controlling agent which provides hydrophilicity which includes a copolymer of unsaturated group-containing carboxylic acid morpholineamide monomer and acrylic acid ester group-containing organosiloxane monomer and a paint composition in which this surface controlling agent is used have been disclosed in Patent Document 2. However, the paint films obtained on coating this paint composition do not retain their staining resistance for a long period of time.
Moreover, a paint composition which includes polydimethylsiloxane-based copolymer, which has a polydimethylsiloxane part and a vinyl polymer chain part, and organosilica sol and the like has been disclosed in Patent Document 3. However, the paint films obtained by coating this paint composition are such that the paint film performance required for coated steel sheet, such as water resistance, bending workability and the like are unsatisfactory.